Hitherto, heater elements have been used, for example, in a glow plug for facilitating start-up of a diesel engine, in a heater for igniting a burner, or in a heater for a gas sensor. Known heater elements include, for example, a metallic sheath heater element including a metallic sheath, heat-resistant insulating powder filled in the sheath, and a heating coil embedded in the powder; and a ceramic heater element including a substrate made of an insulating ceramic material, and a heating resistor made of an electrically conductive ceramic material and embedded in the substrate. Such a heater element is appropriately selected in consideration of the intended use thereof.
A known substrate used in a ceramic heater element contains, for example, a main phase formed of silicon nitride, a grain boundary phase for improving sinterability, and a material for adjusting the thermal expansion coefficient of the substrate so that it becomes nearly equal to the thermal expansion coefficient of a heating resistor for preventing occurrence of cracking due to thermal stress (hereinafter, the material may be referred to as “thermal-expansion-coefficient-adjusting material”). A known grain boundary phase contains, for example, a rare earth element. A known thermal-expansion-coefficient-adjusting material is, for example, a silicide of chromium.
Meanwhile, a known heating resistor used in a ceramic heater element contains, for example, a main phase formed of at least one species selected from among silicide, nitride, and carbide of Mo, and silicide, nitride, and carbide of W; silicon nitride; and a grain boundary phase for improving sinterability. A known grain boundary phase contains, for example, a rare earth element, as in the case of the grain boundary phase of a substrate (see, for example, Patent Document 1).
Such a ceramic heater element is required to consume less electric power and to exhibit excellent rapid heating ability. In order to suppress electric power consumption and to improve rapid heating ability, attempts have been made to improve the structure of such a ceramic heater element; for example, there have been proposed a method in which resistors having different resistances are employed in combination (see, for example, Patent Document 2), and a method in which the cross-sectional area of a heating material is reduced at a heat generation position (see, for example, Patent Document 3).